Thread-safe methods should never be overridden by methods that are not thread-safe. Doing so violates behavioural subtyping, and can result in bugs if the subtype is used in contexts that rely on the thread-safety of the supertype.

Overriding a synchronized method with a method that is not synchronized can be a sign that the thread-safety of the supertype is not being preserved.

class Counter {
  private int count = 0;

  synchronized void increment() {
    count++;
  }
}

// MyCounter is not thread safe!
class MyCounter extends Counter {
  private int count = 0;

  void increment() {
    count++;
  }
}

Note that there are many ways to implement a thread-safe method without using the synchronized modifier (e.g. synchronized statements using explicit locks, or other locking constructs). When overriding a synchronized method with a method that is thread-safe but does not have the synchronized modifier, consider adding @SuppressWarnings("UnsynchronizedOverridesSynchronized") and an explanation.

class MyCounter extends Counter {
  private AtomicInteger count = AtomicInteger();

  @SuppressWarnings("UnsynchronizedOverridesSynchronized") // AtomicInteger is thread-safe
  void increment() {
    count.getAndIncrement();
  }
}